After migrating all mailboxes to Exchange 2016 and Exchange Online it is time to decommission the old Exchange 2010 servers. One of the servers could not be removed and the dreaded “This mailbox database contains one or more mailboxes, mailbox plans, archive mailboxes, or arbitration mailboxes” was shown:

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This typically means that there is some sort of mailbox (arbitration or archive) is still available in the database, causing this issue. Unfortunately using the -Verbose switch did not reveal any other useful information.

Also, when trying to use Remote PowerShell directly (Add-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010, which bypasses RBAC and sometimes help) dit not help either this time.

Note. I was able to find dozens of similar threads on the Internet about this warning, but not a single one applied to my scenario. Ok, I found one where they removed the mailbox database using ADSI Edit, but on the long term that will cause other issues I’m afraid. You can do this, but there are still references to this mailbox database in Active Directory, so stuff will be lingering if you do this. Also, a lot of the threads are about Exchange 2013 and higher, but we were decommissioning Exchange 2010.

Last resort is to look directly into the mailbox database properties in Active Directory using LDP.exe. In LDP, navigate to the mailbox database and open the details (or dump it to a text file). Using the dump a user account was found that still referenced this mailbox database for an archive mailbox:

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The user account was fixed and we were able to remove the mailbox database.